Method of producing silverthallium-indium alloys



Patented May 1, 1951 UNITED STATES ATENT OFFICE METHOD OF PRODUCINGSILVER- THALLIUM-INDIUM ALLOYS Application January 31, 1946, Serial No.644,640

Claims. 1

This invention relates to new and improved silver-thallium-indiumalloys, and, more particularly, to bearings formed from such alloys.

An object of the invention is the production of newsilver-thallium-indium alloys having good anti-friction properties andhigh resistance to corrosion.

Another object of the invention is to provide a method of producingsilver-thallium-indium a1- loys of the above-mentioned type.

Another object of the invention is to provide a method for diffusingthallium and indium into silver. A further object is to provide a methodof producing silver-thallium-indium alloys having desirable bearingproperties and increased resistance to corrosion by means of acombination of electrodeposition and diffusion.

Other objects of the invention will be apparent from the followingdescription taken in connection with the appended claims.

fhe present invention comprises the combination of elements, methods ofmanufacture, and the products thereof brought out and exemplified in thedisclosure hereinafter set forth, the scope of the invention beingindicated in the appended claims.

In the drawing:

Figures 1 to 3 inclusive are graph diagrams showing the distribution ofthallium in silver as is obtained by diffusion treatments of varioustype.

Figures 4 and 5 are cross-sections of two forms of bearingselectroplated with indium and thallium, before the diffusion treatment.

Silver-thallium alloys have various applications of which the mostimportant is in bearings where generally a thin layer of thesilver-thallium alloy is provided upon a suitable backing or base.Considerable difficulty has been experienced, however, in forming suchlayers on a practical and industrial scale. It has been alreadysuggested to codeposit the silver-thallium alloy from a suitableelectrolyte but this procedure was quite slow and cumbersome. Anothersuggestion involved electrodepositing thallium over the silver andsubsequently heating the combination in order to diffuse the thalliuminto the silver. It was found, however, that the resulting penetrationof the thallium into the silver was not deep enough to provide the mostdesirable bearing characteristics. As illustrated graphically in Figure1, when a .0001" film of thallium over silver was heated for six hoursat 500 C. in a hydrogen atmosphere,'starting the furnace at roomtemperature, the resultant diffusion gradient was quite steep. At thedesired depth between 2 and 3 thousandths of an inch, the percentage ofthallium in silver was less than one-fourth of one per cent. Whenthallium alone was diffused into silver a rather crystalline and mottledsurface was left on the surface of the resulting alloy, thus producingan undesirable surface for bearings.

According to the present invention it has been found that a deeper andmore uniform diffusion of the thallium into the silver can beaccomplished by the use of indium in combination with the thallium. Thisnovel method of diffusing thallium into the silver with the aid ofindium leaves a very smooth surface on the silver, which is verydesirable for hearing applications.

An alloy is obtained as shown in Figure 4 by electrodepositing indium IIto a thickness of .00003" upon a plated silver bearing layer 9,electrodeposited on a steel backing 8. Immediately thereafter thalliumi2 is electroplated onto the indium surface to a thickness of .000124'.Then the thallium plated surface is immediately covered withelectrodeposited indium l3 to a thickness of .00001". The indium andthallium are then diffused into the underlying silver 9 by heattreatment in a suitable reducing atmosphere, such as hydrogen, for sixhours at 600 0., starting the furnace from room temperature. Asillustrated graphically in Figure 2, the percentage gradient of thalliumin silver is flattened out considerably and now the percentage ofthallium averages over one per cent at a depth between 2 and 3thousandths of an inch. Thus, the resultant alloy contains four times asmuch thallium as was obtained without the use of indium.

The percentage gradient of thalliumv in silver may be flattened outstill further by prolonging the heat treatment to 12 hours at 600 C. ina hydrogen atmosphere, starting the furnace at room temperature. Figure3 illustrates graphically the results of such prolonged heat treatingwhere the thallium content shows a somewhat smaller percentage deviationbetween a depth of 2 and 3 thousandths of an inch.

The difiusion of thallium and indium into silver may be accomplishedwithout the second indium layer. A plate for diffusion treatment inaccordance with this invention may be prepared as shown in Figure 5where a steel backing 8 has successive electroplated layers of silver 9,indium H and thallium 52.

According to one preferred embodiment of this invention, it is desiredto produce a bearing alloy which shall consist of 2%plus or minusthallium in silver, this percentage to persist to a depth of at least.002" below the unworn bearing surface. As shown in Figure 4, such analloy is obtained by depositing thallium l2 to a thickness of .0002 on asilver plated bearing layer 9 upon which induim II has beenelectroplated to a thickness of .00002" immediately prior to thedeposition of the thallium. The thallium plated surface is immediatelycovered with electrodeposited indium I3 to a thickness of .0002". Theindium and thallium are then diffused into the underlying silver by heattreatment in a hydrogen atmosphere at about 500 to 600 C. for a periodof about 6 to 12 hours, starting the furnace from room temperature.

By varying the thickness of the thallium and indium deposits upon thesilver bearing layer and by varying the furnacing temperatures and timeperiods, silver alloys can be produced by the diffusion method of thepresent invention which contain up to about thallium and up to about 10%of indium.

The preferred composition embodying the invention expressed inpercentages by weight of the constituents comprises:

Per cent Thallium About 2.5 Indium About .5 Silver Balance In preparingthe silver base upon which the thallium and indium are to beelectrodeposited it is essential that the silver be strongly bonded tothe steel so that the bond will not fail under the chisel test afterhaving been subjected to a temperature of 600 C. for six hours. Thesteel backing is properly cleaned, after which it is subjectedsuccessively to a nickel strike and a silver strike, and then given afinal silver plate. As silver plating methods and baths are well knownto those skilled in the art no detailed description of such methods isbelieved to be necessary.

It is essential that the work be silver plated oversize enough that itmay be machined to proper dimension after plating. The plated silvermust be machined to .00024 under size of the final dimension sincesubsequent plating operations will add .00024" to the final thickness ofthe bearing material, these dimensions being applicable to the preferredembodiment outlined in the foregoing.

The silver plated work, machined to the proper size, is then thoroughlydegreased in a suitable volatile solvent either by handor in a suitabledegreaser. After masking to prevent plating where not desired, the firstindium layer is applied according to the procedures outlined in the GrayPatent 1,935,630 and in the Murray et a1. Patent 1,965,251.

The thalium deposit must be immediately applied without delay. The work,having been rinsed from the indium plating bath, is immediately placedin the thallium bath before any water-break appears on the surface ofthe work.

The final indium deposit must be applied before the work shows anywater-break after the final rinse following the thallium plating. Thefinal indium plate is applied exactly like the first indium plate, andto the same thickness; however, the plating time may be different sinceindium apparently deposits from a cyanide bath at a difierent rate onthallium than it does on silver. Alternatively, as previously stated,the final in- 4 dium plate may be only about one-third as thick as thefirst indium plate.

After the application of the final indium plate the work should be blowndry and the masking removed. The indium plate must not be touched andmust be protected from scratches. The plated work is placed in ahydrogen furnace at room temperature and the temperature then raised to600 C. and maintained for six hours, after which the work is removed toa cooling chamber, and allowed to cool before being removed.

A thallium electroplating bath may be made up in the following manner:

Thallium metal, 25 grams per liter (3.3 ounces per gallon) Freeperchloric acid, 10 grams per liter (1.3

ounces per gallon) Peptone, 10 grams per liter (1.3 ounces per gallon)Cresylic acid, 2 to 3 milliliters per liter (7.5 to 11 milliliters pergallon) lhe bath is prepared by shaving up the thallium metal anddissolving it in a slight excess of perchloric acid and warming under afume hood until the metal is all dissolved. After adding the peptone,the level is adjusted and the free perchloric acid is titrated withstandard NaOH solution. The free perchloric acid is then adjusted andthe cresylic acid is added.

The thallium plate rate needs to be checked only occasionally. Theindium plating rate, however, should be checked frequently, and aftereach bath correction; and further, the plating rate of indium should bechecked on thallium and on silver both, as the plating rate of indiumwill vary with the base metal. Plating rate tests should be made using athin copper dummy cathode silver plated to about 0.002 inch and of thesame size and shape as the work.

It is essential that the distribution of both indium and thallium beuniform over the surface of the Work. Correct distribution is usuallyobtained only by cut and try methods. Distribution Varies principallywith anode size and location, and with agitation of the work or thebath.

In furnacing the bearings with the plated layers of indium and thallium,it is desirable to start the furnacing at room temperature so thatdiffusion of the plated metals will begin before their melting pointsare reached. This prevents sudden melting and running off of thethallium and indium coating as would be the case if the bearings wereplaced directly in the furnace at high temperature.

The present invention provides a uniform silver-thallium bearing surfaceand a method of producing the same which avoids the difficultiesencountered in directly co-depositing silver and thallium. According tothe present invention the powerful wetting effect of indium is utilizedin successfully driving the thallium into the silver during theheat-treatment in a hydrogen atmosphere while previous attempts todiffuse thallium. without indium, into silver failed.

The resulting silver-indium-thallium bearing of this invention possessesa high fatigue resistance, excellent corrosion and wear resistance, asmooth surface with a low coefficient of friction and a comparativelyhigh thermal conductivity. The method of the invention makes possiblethe formation of thallium-silver hearings in which the thallium isdiffused in a fairly uniform percent'age to the desired depth under thebearing surface.

While the present invention, as to its objects and advantages, has beendescribed herein as carried out in specific embodiments thereof,variations and modifications may be resorted to by those skilled in theart without departing from the principles of the invention. Thus, it hasbeen found that the method of the invention may be applied with equal orsimilar results to producing copper base bearings wherein thallium andindium are diifused into the copper much in the same way as with silverbase bearings. All of these variations and modifications are consideredto be within the true spirit and scope of the present invention, asdisclosed in the foregoing description and defined by the appendedclaims.

What is claimed is:

1. The method of forming a bearing alloy which comprises electroplatingsuccessive layers of indium, thallium and indium over a silver base andsubjecting the resulting combination to heat treatment for about 6 to 12hours at a temperature of about 500 to 600 C. to cause diffusion of thethallium and indium into the silver and the formation of a thalliumindium silver alloy.

2. The method diffusing thallium into silver which compriseselectrodepositing successive layers of indium and thallium upon a silverbase, and subjecting the resulting combination to heat treatment in areducing atmosphere for about 6 to 12 hours at a temperature of about500 to 600 C.

3. The method of diffusing thallium and indium into silver whichcomprises electrodepositing successive layers of indium, thallium andindium onto a silver base and subjecting the resulting combination toheat treatment in a hydrogen atmosphere at a temperature of about 600 C.for about 6 hours, starting the furnace at room temperature.

4. The method of forming a bearing alloy which comprises electroplatinga layer of indium having a thickness of about .00002 inch to about.00003 inch upon a silver base, electroplating a layer of thalliumhaving a thickness of about .00012 inch to about .0002 inch upon saidindium layer, and heat treating the resulting plated silver base at atemperature of about 500 to 600 C. for about 6 to 12 hours.

5. The method of forming a bearing alloy which comprises electroplatinga layer of indium having a thickness of about .00002 inch to about.00003 inch upon a silver base, electroplating a layer of thalliumhaving a thickness of about .00012 inch to about .0002 inch upon saidindium layer, electroplating a layer of indium having a thickness ofabout .00001 inch to about .00002 inch upon the thallium layer, and heattreating the resulting plated silver base at a temperature of about 500to 600 C. for about 6 to 12 hours.

JAMES M. BOOE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Hensel, Silver-ThalliumAntifriction Alloys, Metal Technology, Technical Publication No. 1930.pp. 1-14, October 1945.

1. THE METHOD OF FORMING A BEARING ALLEY WHICH COMPRISES ELECTROPLATINGSUCCESSIVE LAYERS OF INDIUM, THALLIUM AND INDIUM OVER A SILVER BASE ANDSUBJECTING THE RESULTING COMBINATION OF HEAT TREATMENT FOR ABOUT 6 TO 12HOURS AT A TEMPERATURE OF ABOUT 500 TO 600* C. TO CAUSE DIFFUSION OF